Initially, domain names were restricted to ASCII characters.
A system was introduced in 2003 for internationalized domain names (IDN).
This system is called Internationalizing Domain Names for Applications,
or IDNA2003 for short.
This mechanism supports IDNs by means of a client software transformation
into a format known as Punycode.
A revision of IDNA was approved in 2010 (IDNA2008).
This revision has a number of incompatibilities with IDNA2003.

The incompatibilities force implementers of client software,
such as browsers and emailers,
to face difficult choices during the transition period
as registries shift from IDNA2003 to IDNA2008.
This document specifies a mechanism
that minimizes the impact of this transition for client software,
allowing client software to access domains that are valid under either system.